1. Field of the Invention
This invention relates to electromagnetic acoustic transducers (hereinafter referred to as EMATS) for inspecting the integrity of metallic components, for example pipelines, by ultrasonics.
2. Summary of the Prior Art
Conventional EMATs interact with test materials by the joint action of a steady magnetic field, often produced by one or more permanent magnets, and a transient high frequency magnetic field, produced by an electrical winding. The interaction of the EMAT with the test specimen is usually at A maximum when the gap between the active components of the transducer and the test material is at a minimum.
However, EMATs are subjected to wear if moved while in contact with a test material.
Consequently EMATs to be moved along the surface of a test material require the provision of a protective layer or wear plate between the active components of the EMAT and the surface. This layer is subject to conflicting requirements. Wear resistance improves as layer thickness increases, but the acoustic performance of the EMAT decreases as the layer thickness increases, and is dependent upon the material properties and geometry of the protective layer.
The materials of protective layers usually incorporated in EMATs are chosen to have a negligible interaction with the EMAT, their presence having no other effect on acoustic performance than that associated with introducing an unfilled gap between the active face of the EMAT and the test material. Protective layers made from these materials are typically very thin, because EMAT acoustic performance falls very sharply as the gap increases. Since the material is thin, the lifetime of the wear layer can be short if the abrasion it experiences is particularly severe, for example in long distance high speed inspection of a pipe wall from an internal inspection vehicle or pipeline pig.
The use of electrically conductive and/or ferromagnetic material for the protective plate has heretofore been outlawed because the acoustic performance of the EMAT is severely reduced if a plate of such material is interposed between the EMAT and the material under test.
More particularly the presence of such a plate significantly reduces the penetration of the high frequency magnetic field from the EMAT into the test material due to the electrical skin depth phenomenon, and the DC magnetic field in the test material is reduced due to the removal of D C Flux from the test material by alternative closure paths.